Polyesters such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are known to possess good chemical stability, physical and mechanical strength, durability, heat resistance, chemical resistance, weather resistance and electrical insulation property; and, therefore, have been widely used in manufacturing various articles including medical devices, capacitors, packaging and labelling materials, photographic film and magnetic recording media.
Recently, polyester films have been increasingly used as paper substitutes in a variety of applications. However, conventional polyester films are inferior to paper in clarity, color, density and rigidity and, therefore, extensive attempts have been made to develop a polyester film having paper-like softness while maintaining the mechanical strength, heat resistance, durability and the other desirable properties of the polyester. For example, Japanese Patent Laid-open Publication No. 50625/1983 discloses a method for preparing a polyester film by incorporating a foaming agent therein; and Japanese Patent Laid-open Publication No. 49648/1982 offers a method for preparing a polyester film with improved surface properties by blending a polyolefin resin with a polyester resin and extending the resultant mixture to form pores on the surface as well as inside of the film obtained therefrom, and optionally incorporating therein a thermal stabilizer.
However, such attempts have their own drawbacks and problems. In case a foaming agent is incorporated in a polyester film, the pores formed tend to be unevenly dispersed and physical properties of the film are difficult to control. Further, in case a polyolefin resin is blended with a polyester resin, the properties of the resulting polymer film are apt to deteriorate due to poor heat resistance of the polyolfin component. Also, since a polyolefin tends to generate and accumulate static electricity, the film prepared from a polyester blended with a polyolfin may suffer from electrostatic and low printability problems, limiting its usage as a paper substitute.
The present invention is based on a discovery that the surface property of a film is intimately related to the size, shape and distribution of pores present in the film. If the distribution of pores in the film is not suitably designed, then a polymer film having paper-like softness, desirable surface properties and good processability cannot be obtained.